Kinetics of the Decomposition and Hydrogen Reduction of Nitric Oxide on Niobium, Nickel and Platinum Filaments at High Temperatures and Low Pressures

Abstract

NO decomposition kinetics were studied with ultra-high vacuum techniques on niobium, nickel and platinum wire filaments at high temperatures with pressures between —1 and 100 m-2. Niobium readily decomposed NO with first-order kinetics, nitrogen and oxygen being deposited on the surface, resulting in a progressive loss of filament activity. Nitrogen was desorbed from nickel, which otherwise behaved similarly to niobium. Excess H2 affected the kinetics of NO reaction on these metals mainly by reaction with and subsequent removal of surface contaminants rather than by a direct reaction with NO. Platinum did not observably decompose NO at low pressures except in the presence of excess H2. Above ~1300 K the rate appeared to be determined by H2 atomisation, but at lower temperatures a reaction between H2 and NO may have been occurring. The rate-limiting step for this reaction may be the decomposition of H2 and addition of H atoms to adsorbed NO. The pre-exponential term for this reaction indicated that the transition state was mobile on the surface.